The present invention relates generally to wireless communication systems and, more particularly, to a method and apparatus for performing a signal detection and a rake receiver finger assignment in a wireless communication system.
Communication systems that utilize coded communication signals are known in the art. One such system is a direct sequence Code Division Multiple Access (DS-CDMA) cellular communication system, such as set forth in the Telecommunications Industry Association Interim Standard 2000 (TIA IS-2000) herein after referred to as IS-2000. In accordance with IS-2000, the coded communication signals used in the DS-CDMA system includes signals that are transmitted in a common channel, typically a 1.25 MHz bandwidth common channel, between mobile stations (MS) and base transceiver stations (BTS) located at the base sites (BS) of a wireless communication system. Each DS-CDMA signal includes, inter alia, a pseudorandom noise (PN) binary code associated with a particular base transceiver station and a PN sequence associated with a particular mobile station.
Another such system is a Universal Mobile Telecommunications System (UMTS) cellular communication system, such as set forth in IMT-2000. Like the IS-2000 system, UMTS is a DS-CDMA based technology that transmits signals in a common channel between mobile stations and BTS""s.
During a typical wireless communication, a mobile station communication signal is supported by the BTS associated with the coverage area in which the mobile station is traveling. Such movement typically results in fading of the mobile communication signal due to multipath propagation of the transmitted signal. As is known, multipath propagation results from the reflections of the transmitted mobile communication signal off of nearby scatterers such as buildings. These reflections produce replicas, typically referred to as multipath replicas, of the originally transmitted signal. The multipath replicas, or signals, generally arrive at the BTS at various energy levels, generally below the originally transmitted mobile communication signal, and at various times. The various arrival times are expressed as an offset in time from a path with no propagation delay and are therefore referred to as time offsets. Additionally, multipath replicas are sometimes referred to as rays having offset ray energies at the various time offsets.
A multipath signal searcher assembly located at the BTS initially searches received signals across a range of time offsets to determine whether a transmitted mobile station communication signal has sufficient power for subsequent demodulation by a BTS RAKE receiver. Generally, an initial signal, for example a preamble signal, is transmitted by the mobile station to facilitate the initial signal search. In addition, prior to sending a message signal (voice or data), a mobile station transmits information to facilitate a call set-up or to register its location with a base station so that it may be notified of incoming calls. For example, in a UMTS system, the mobile station sends an origination or registration message to a base station via a UMTS Random Access Channel (RACH). When the mobile station is ready to transmit the information, it sets a transmit-power level to an initial value and then transmits the preamble signal containing a 4096 chip code sequence that is recognizable by the multipath signal searcher at the base station.
Upon receipt of the mobile station transmission, the multipath signal searcher assembly searches for the preamble code sequence. Once the code sequence is detected, the prior art search engine algorithm in the multipath signal searcher assembly first selects the offset ray energy having the highest power value and then compares that offset ray energy to a predefined threshold. Offset ray energies that do not have the highest power value, or offset energies which do have the highest power but fall below the predefined threshold, are discarded. If the maximum offset ray energy of the set of these ray energies is above the predefined threshold, then the base station replies to the mobile station with an acknowledgment on an Access Indication Channel (AICH). Upon receipt of the acknowledgment, the mobile station replies by transmitting its message frame. The multipath signal searcher assembly then forwards the time offset and frequency information a finger manager to initialize RAKE receiver fingers for demodulation of the mobile station""s message frame.
If however, upon receipt of the code sequence, the multipath signal searcher assembly determines that the mobile station""s transmit power is insufficient to support reliable message transmission, it does not send an acknowledgment. If the mobile station does not receive an acknowledgment after a fixed period of time, it increases its transmit power and retransmits the preamble. The process repeats until the mobile station""s transmit power increases enough to cause receipt of a maximum energy ray above a predefined threshold by the multipath signal searcher, or a maximum number of preamble transmissions is reached.
As the mobile station increases it""s transmit power, uplink signal interference increases. This is particularly damaging to wireless communication systems such as CDMA where system capacity is directly related to signal interference. Moreover, incorrect acknowledgments by the base station result in subsequent unreliable message transmissions from the mobile station which require the mobile station to restart the entire access procedure at a later time. This ultimately delays the reception of the message data, causes additional interference on the uplink channels, requires additional access channel hardware resources at the base station, causes additional congestion on the access channel, and reduces mobile battery life.
There are two problems associated with the prior art search engine algorithm for detection and demodulation. First, because the sensitivity of the base station receiver is limited by the predefined threshold used by the multipath signal searcher assembly, secondary rays which do not exceed the predefined threshold but which contain significant signal power are not tracked and decoded. Second, because the base station acknowledgment is triggered when one high powered ray, by itself, exceeds the predefined threshold, the mobile station often continues to increase its transmit power unnecessarily, even though a combination of secondary, or lower power, rays could be utilized for reliable message decoding.